We have investigated ultrafast carriers dynamics in crystalline silicon nano-pillars structure using a pump-probe reflectivity method with 800 nm, 150 fs laser pulses and fluence in the range of {17 - 170} mJ/cm^2. Dimensions of the structure allow us to eliminate contribution from the diffusion process to the relaxation dynamics of the excited carriers. Strong intensity-dependent time-resolved reflectivity change, ΔR(τ_d), was monitored in the submelting regime. At strong pumping ΔR(τ_d) can reach up to 8%, a several times higher than for a bulk silicon. From the measurements we deduced recombination time of up to 10 ns at carriers density of ~1 x 10^20 cm^-3, while electron-phonon interaction occurs during 350-400 fs and it is independent of the concentration.
Regular arrays of InP nano pillars have been fabricated by low energy Electron Cyclotron Resonance (ECR) Ar+ ion irradiation on InP(111) surface. Several scanning electron microscopy (SEM) images have been utilized to invetsigate the width, height, and orientation of these nano pillars on InP(111) surfaces. The average width and length of these nano-pillars are about 50 nm and 500 nm, respectively. The standing angle with respect to the surface of the nano-pillars depend on the incidence angle of the Ar ion irradiation during the fabrication process. Interestingly, the growth direction of the …
We report on a spectroscopic study of the spin-wave eigen-modes inside an individual normally magnetized two layers circular nano-pillar (Permalloy$|$Copper$|$Permalloy) by means of a Magnetic Resonance Force Microscope (MRFM). We demonstrate that the observed spin-wave spectrum critically depends on the method of excitation. While the spatially uniform radio-frequency (RF) magnetic field excites only the axially symmetric modes having azimuthal index $\ell=0$, the RF current flowing through the nano-pillar, creating a circular RF Oersted field, excites only the modes having azimuthal index $\…
In this paper we numerically conduct micromagnetic modelling with an expended micromagnetic model that includes the spin torque term and an impedance model to investigate methods to replace external field sources with internal ones and to investigate its tuneability on nanopillar geometries. We present results for three methods: interlayer coupling, large perpendicular anisotropy and magnetostatic coupling. The internal field sources are evaluated as function of frequency shift with current, its dependency on temperature and are tested against analytical predictions.
Dynamic quantities related to hysteresis have been measured in micromagnetic simulations of single-domain nanoscale magnets at nonzero temperature. The hysteresis-loop area and magnetization-field correlation display the characteristics of resonance, and the resonance frequency is found to be temperature dependent. The period-averaged magnetization displays symmetry breaking at high frequencies.
We describe the quantitative refinement of nanoparticle structures from gold nanoparticles probed by ultrafast electron crystallography (UEC). We establish the equivalence between the modified radial distribution function employed in UEC and the atomic pair distribution function (PDF) used in x-ray and neutron powder diffraction analysis. By leveraging PDF refinement techniques, we demonstrate that UEC data are of sufficient quality to differentiate between cuboctahedral, decahedral and icosahedral nanoparticle models. Furthermore, we identify the signatures of systematic errors that may occur…
Using a three-dimensional focused-ion beam lithography process we have fabricated nanopillar devices which show spin transfer torque switching at zero external magnetic fields. Under a small in-plane external bias field, a field-dependent peak in the differential resistance versus current is observed similar to that reported in asymmetrical nanopillar devices. This is interpreted as evidence for the low-field excitation of spin waves which in our case is attributed to a spin-scattering asymmetry enhanced by the IrMn exchange bias layer coupled to a relatively thin CoFe fixed layer.
Ultrafast degenerate pump-probe experiments performed on a free standing film of double walled carbon nanotubes near the first metallic transition energy of outer tube show ultrafast ($97 fs$) photobleaching followed by a photo-induced absorption with a slow relaxation of $1.8 ps$. Femtosecond closed and open aperture z-scan experiments carried out at the same excitation energy show saturation absorption and negative cubic nonlinearity. From these measurements, real and imaginary part of the third order nonlinear susceptibility are estimated to be Re($χ^{(3)}) \sim -2.2 \times 10 ^{-9} esu$ an…
Microwave treated water soluble and amide functionalized single walled carbon nanotubes have been investigated using femtosecond degenerate pump-probe and nonlinear transmission experiments. The time resolved differential transmission using 75 femtosecond pulse with the central wavelength of 790 nm shows a bi-exponential ultrafast photo-bleaching with time constants of of 160 fs (130 fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes. Open and closed aperture z-scans show saturation absorption and positive (negative) nonlinear refraction for water soluble (amide functio…
We report the studies of ultrafast electron nanocrystallography on size-selected Au nanoparticles (2-20 nm) supported on a molecular interface. Reversible surface melting, melting, and recrystallization were investigated with dynamical full-profile radial distribution functions determined with sub-picosecond and picometer accuracies. In an ultrafast photoinduced melting, the nanoparticles are driven to a non-equilibrium transformation, characterized by the initial lattice deformations, nonequilibrium electron-phonon coupling, and upon melting, the collective bonding and debonding, transforming…
Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque Magnetic Random-Access Memory (STT-MRAM) beyond the sub-20 nm technology node whilst retaining thermal stability of the storage layer magnetization. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. Here, the magnetism of a FeCoB / NiFe PSA-STT-MRAM nano-pillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temper…
The full control of the fundamental photophysics of nanosystems at frequencies as high as few THz is key for tunable and ultrafast nano-photonic devices and metamaterials. Here we combine geometrical and ultrafast control of the optical properties of halide perovskite nanoparticles, which constitute a prominent platform for nanophotonics. The pulsed photoinjection of free carriers across the semiconducting gap leads to a sub-picosecond modification of the far-field electromagnetic properties that is fully controlled by the geometry of the system. When the nanoparticle size is tuned so as to ac…
The femtosecond evolution of the electronic temperature of laser-excited gold nanoparticles is measured, by means of ultrafast time-resolved photoemission spectroscopy induced by extreme-ultraviolet radiation pulses. The temperature of the electron gas is deduced by recording and fitting high-resolution photo emission spectra around the Fermi edge of gold nanoparticles providing a direct, unambiguous picture of the ultrafast electron-gas dynamics. These results will be instrumental to the refinement of existing models of femtosecond processes in laterally-confined and bulk condensed-matter sys…
We demonstrate a viable approach to fabricating ultrafast axial nano-oscillators based on carbon nanoscrolls (CNSs) using molecular dynamics simulations. Initiated by a single-walled carbon nanotube (CNT), a monolayer graphene can continuously scroll into a CNS with the CNT housed inside. The CNT inside the CNS can oscillate along axial direction at a natural frequency of 10s gigahertz (GHz). We demonstrate an effective strategy to reduce the dissipation of the CNS-based nano-oscillator by covalently bridging the carbon layers in the CNS. We further demonstrate that, such a CNS-based nano-osci…
We study the ultrafast dynamics of surface electromagnetic waves photogenerated on aluminum film perforated with subwavelength holes array by means of transient photomodulation with 100 fs time resolution. We observed a pronounced blueshift of the resonant transmission band that reveals the important role of plasma attenuation in the dynamics and that is inconsistent with plasmon-polariton mechanism of extraordinary transmission. The transient photomodulation spectra were successfully modeled within the Boltzmann equation approach for the electron-phonon relaxation dynamics, involving non-equi…
To verify the exact underlying mechanism of ultrafast vortex-core reversal as well as the vortex state stability we conducted numerical calculations of the dynamic evolution of magnetic vortices in Permalloy cylindrical nanodots under an oscillating in-plane magnetic field over a wide range of the frequency and amplitude. The calculated results reveal different kinds of the non-trivial dynamic responses of vortices to the driving external field. In particular, the results offer insight into the 10 ps scale underlying physics of the ulrafast vortex-core reversals driven by small amplitude (~10 …
Understanding microscopic processes in materials and devices that can be switched by light requires experimental access to dynamics on nanometer length and femtosecond time scales. Here, we introduce ultrafast dark-field electron microscopy, tailored to map the order parameter across a structural phase transition. We track the evolution of charge-density wave domains in 1T-TaS2 after ultrashort laser excitation, elucidating relaxation pathways and domain wall dynamics. The unique benefits of selective contrast enhancement will inspire future beam shaping technology in ultrafast transmission el…
We report a novel phenomenon in carbon nanotube (CNT) based devices, the transphonon effects, which resemble the transonic effects in aerodynamics. It is caused by dissipative resonance of nanotube phonons similar to the radial breathing mode, and subsequent drastic surge of the dragging force on the sliding tube, and multiple phonon barriers are encountered as the intertube sliding velocity reaches critical values. It is found that the transphonon effects can be tuned by applying geometric constraints or varying chirality combinations of the nanotubes.
We present a numerical model allowing to determine the electron and lattice temperature dynamics in a gold nanoparticle under subpicosecond pulsed excitation, as well as that of the surrounding medium. For this, we have used the electron-phonon coupling equation in the particle with a source term linked with the laser pulse, and the ballistic-diffusive equations for heat conduction in the host medium. Our results show that the heat transfer rate from the particle to the matrix is significantly smaller than the prediction of Fourier's law. Consequently, the particle temperature rise is much lar…
Characterizing and controlling the out-of-equilibrium state of nanostructured Mott insulators hold great promises for emerging quantum technologies while providing an exciting playground for investigating fundamental physics of strongly-correlated systems. Here, we use two-colour near-field ultrafast electron microscopy to photo-induce the insulator-to-metal transition in a single VO2 nanowire and probe the ensuing electronic dynamics with combined nanometer-femtosecond resolution. We take advantage of a femtosecond temporal gating of the electron pulse mediated by an infrared laser pulse, and…
The control of optically driven high-frequency strain waves in nanostructured systems is an essential ingredient for the further development of nanophononics. However, broadly applicable experimental means to quantitatively map such structural distortion on their intrinsic ultrafast time and nanometer length scales are still lacking. Here, we introduce ultrafast convergent beam electron diffraction (U-CBED) with a nanoscale probe beam for the quantitative retrieval of the time-dependent local distortion tensor. We demonstrate its capabilities by investigating the ultrafast acoustic deformation…
We report ultrafast electron dynamics in gold nanorods investigated using 80 fs laser pulses centered at 1.57 eV. Five types of nanorod colloidal suspensions in water having their longitudinal surface plasmon peak (ELSP) on either side of the laser photon energy (EL) have been studied. For ELSP>EL, photo-induced absorption with single decay time constant is observed. On the other hand, for ELSP<EL, photo-bleaching is observed having biexponential decay dynamics; the faster one between 1-3 ps and slower one between 7ps to 22ps both of them increasing almost linearly with the difference |EL-ELSP…
We present results of wavelength-dependent ultrafast pump-probe experiments on micelle-suspended single-walled carbon nanotubes. The linear absorption and photoluminescence spectra of the samples show a number of chirality-dependent peaks, and consequently, the pump-probe results sensitively depend on the wavelength. In the wavelength range corresponding to the second van Hove singularities (VHSs), we observe sub-picosecond decays, as has been seen in previous pump-probe studies. We ascribe these ultrafast decays to intraband carrier relaxation. On the other hand, in the wavelength range corre…
We describe two general procedures for fabricating microstructures with large overhangs and high aspect-ratio support pillars. The first method uses a static angled dry etch on micro- or nano-pillars to create an initial overhang, followed by wet etching for further erosion. The second method uses a time-dependent angled etch on a flat plane patterned with protective resin, to reduce the number of lithography steps needed to make these objects. The time-dependent dry etch is again followed by a wet etch. For the second method we derive a formula that provides the rate at which the attack angle…
Here we propose a new computational approach to light-to-matter interactions in silicon nanopillars, which simulates heat generation and propagation dynamics occurring in continuous wave laser processing over a wide temporal range (from 1 fs to about 25 hours). We show that visible light can be exploited to selectively crystallize internal region of the pillars, which is not possible by conventional treatments. A detailed study on lattice crystallization and reconstruction dynamics reveals that local heating drives the formation of secondary antennas embedded into the pillars, highlighting the…
Numerical integration of a stochastic Landau-Lifshitz-Gilbert equation is used to study dynamic processes in single-domain nanoscale magnets at nonzero temperatures. Special attention is given to including thermal fluctuations as a Langevin term, and the Fast Multipole Method is used to calculate dipole-dipole interactions. It is feasible to simulate these dynamics on the nanosecond time scale for spatial discretizations that involve on the order of 10000 nodes using a desktop workstation. The nanoscale magnets considered here are single pillars with large aspect ratio. Hysteresis-loop simulat…
We present a general formalism of ultrafast diffractive voltammetry approach as a contact-free tool to investigate the ultrafast surface charge dynamics in nanostructured interfaces. As case studies, the photoinduced surface charging processes in oxidized silicon surface and the hot electron dynamics in nanoparticle-decorated interface are examined based on the diffractive voltammetry framework. We identify that the charge redistribution processes appear on the surface, sub-surface, and vacuum levels when driven by intense femtosecond laser pulses. To elucidate the voltammetry contribution fro…
